Echo processing devices for single-channel or multichannel communication systems

ABSTRACT

An echo processing technique for attenuating echo components of a direct signal X 1   n  in a return signal Y 2   n . A receive gain Gr n  and a send gain Ge n  are calculated. The receive gain Gr n  is applied to the direct signal and an input signal X 2   n  is produced and emitted into an echo generator system. The send gain Ge n  is applied to an output signal Y 1   n  from the echo generator system and the return signal Y 2   n  is produced. A coupling variable COR is calculated which is characteristic of the acoustic coupling between the direct signal X 1   n  or the input signal X 2   n  and the output signal Y 1   n . The receive gain Gr n  and the send gain Ge n  are calculated on the basis of the coupling variable.

RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 USC 371 ofInternational Application PCT/FR2003/001874, filed on 18 Jun. 2003.

FIELD OF THE INVENTION

The field of the present invention is that of communications. Theinvention relates more particularly to variable-gain and/or adaptivefiltering acoustic echo processing devices for attenuating echocomponents of a direct signal in a return signal. The invention appliesto single-channel and multichannel communications systems.

BACKGROUND OF THE INVENTION

Acoustic echoes occur primarily in certain types of communication inwhich a remote user terminal comprises one or more directionalmicrophones and one or more loudspeakers instead of an earpiece.Examples include audioconference equipment and hands-free telephones,such as mobile telephones. The source of the echoes is simple: failingspecial precautions, sound emitted by the loudspeaker(s) is reflectedmany times (from walls, the ceiling, etc.), constituting as manydifferent echoes which are picked up by the microphone(s) on the sameterms as wanted speech. Thus the combination of the loudspeaker(s), themicrophone(s), and their physical environment constitutes an echogenerator system.

The acoustic echo problem has been the subject of much research, both inthe case of single-channel systems (one microphone and one loudspeaker)and in the case of multichannel systems (a plurality of microphones anda plurality of loudspeakers). The echo problem in the multichannelsituation is similar to that in the single-channel situation except thatall possible acoustic couplings between the various microphones andloudspeakers must be considered.

The echo processing techniques most widely used include echo suppressiontechniques using gain variation and echo cancellation techniques usingadaptive filtering.

In a variable-gain echo suppression system, a receive gain is applied tothe signal for application to the loudspeaker (the direct signal at theinput of the echo generator system) and a send gain is applied to thesignal coming from the microphone (at the output of the echo generatorsystem), forming the return signal. An echo suppression system of thistype is described in French Patent No. 2 748 184.

Receive voice activity detectors (RVAD), send voice activity detectors(SVAD), and double speech detectors (DSD) typically supply the necessaryinformation to the modules that calculate the send and receive gains.Thus when the remote party is speaking (detected by the RVAD), the sendgain is reduced to attenuate the echo. If the local party begins tospeak (detected by the SVAD), this constraint on the send gain isremoved and the receive gain is reduced. In the event of double speech(both parties speaking simultaneously, detected by the DSD), either acomparator determines which speaker is louder and gives priority to thatspeaker's sending direction or an intermediate setting of the send andreceive gains is established.

In an acoustic echo canceller (AEC) using adaptive filtering, anidentification filter estimates the acoustic coupling between theloudspeaker and the microphone and generates a signal that is used tocancel the echo. The identification filter is conventionally aprogrammable finite impulse response filter whose coefficients need tobe adapted by a predetermined algorithm for updating coefficients usingan adaptation step. The coefficients are adapted on the basis of thesignal to be applied to the loudspeaker. An echo canceller of this typeis described in French Patent No. 2 738 695.

A variable gain echo suppression system is often combined with an echocanceller to eliminate the residual echo that remains after echocancellation.

However, the above-mentioned echo processing systems have the drawbackthat they are not able to take account of variations in the acousticcoupling between the loudspeaker and the microphone if those variationsare independent of the signal applied to the loudspeaker.

This is the case, for example, if there is an external facility foradjusting the sound level reproduced by the loudspeaker (for example bymeans of a potentiometer). Any variation in the reproduced sound levelmodifies the acoustic coupling between the loudspeaker and themicrophone and therefore the echo(es) picked up by the microphone. Theecho processing system takes account only of the signal that is appliedto the loudspeaker, and not of the sound that is actually reproduced bythe loudspeaker, and is therefore unable to take this kind ofmodification of the acoustic coupling into account in its calculationprocess.

For example, if the sound reproduction level is reduced after the systemhas been initialized with a maximum sound level setting, in a doublespeech situation the remote speech emitted by the loudspeaker may bebroken up or truncated.

Similarly, if the microphone and the loudspeaker in the communicationsterminal being used are physically independent of each other, thedistance between them may be varied, which varies the acoustic couplingbetween the loudspeaker and the microphone, with the same consequences.

The problem is the same in a multichannel situation except that itgeneralized to the multiple couplings between the various microphonesand loudspeakers.

SUMMARY OF THE INVENTION

One particular object of the present invention is to remedy thedrawbacks of prior art echo processing systems described hereinabove.

To this end, in a first aspect, the present invention provides an echoprocessing device for attenuating echo components of a direct signal X1n in a return signal Y2 n, said device comprising:

-   -   means for calculating a receive gain Gr_(n) and a send gain        Ge_(n);    -   first gain application means for applying the receive gain        Gr_(n) to the direct signal and producing an input signal X2 n        emitted into an echo generator system; and    -   second gain application means for applying the send gain Ge_(n)        to an output signal Y1 n from the echo generator system and        producing the return signal Y2 n.

According to an embodiment of the invention, this echo processing deviceis noteworthy in that it further comprises means for calculating acoupling variable COR characteristic of the acoustic coupling betweenthe direct signal X1 n or the input signal X2 n and the output signal Y1n and in that said gain calculation means are adapted to calculate thereceive gain Gr_(n) and the send gain Ge_(n) on the basis of saidcoupling variable.

Taking account in the device of the real acoustic coupling between theloudspeaker and the microphone when controlling the variation of thereceive and/or send gain applied automatically adapts the sound qualityof the sent signal and the received signal as a function of changes inthe acoustic environment of the echo processing device and the relativeposition of the transducers (loudspeaker(s), microphone(s)) and as afunction of the sound reproduction level chosen by the user, forexample.

According to one particular feature of the invention, the echoprocessing device comprises means for estimating the instantaneous powerof the direct signal X1 n or the input signal X2 n and the instantaneouspower of the output signal Y1 n. The gain calculation means are adaptedto calculate the receive gain Gr_(n) and the send gain Ge_(n) on thebasis of a variable G determined as a function of the estimated power ofthe direct signal or the input signal and the estimated power of theoutput signal and as a function of the coupling variable COR, inaccordance with the following equation:

$G = \frac{P\; 2\; n}{{P\; 2\; n} + {{{COR} \cdot P}\; 1\; n}}$

where P1 n and P2 n are respectively an estimate of the power of thedirect signal X1 n or the input signal X2 n and an estimate of the powerof the output signal Y1 n at the time concerned.

The term “COR·P1 n” in the expression for the variable G represents theenergy of the sound actually picked up by the microphone, and thereforetaking into account all external adjustments that are not “seen” by thesystem (for example the sound reproduction level). The variable Gtherefore varies automatically as a function of real changes inloudspeaker/microphone acoustic coupling and the send and receive gainsare therefore adapted automatically.

In a second aspect, the invention provides an echo canceller forattenuating, in an output signal Y1 _(n), echo components of an inputsignal X2 n emitted into an echo generator system, said devicecomprising:

-   -   a finite impulse response identification filter whose response        is representative of the response of the echo generator system,        receiving the input signal X2 n at its input and generating a        filtering signal Sn;    -   subtraction means receiving at an input a signal Y3 n from the        echo generator system, at least one component of which is a        response of the echo generator system to the input signal X2 n,        and the filtering signal Sn, and adapted to subtract the        filtering signal Sn from the signal Y3 n and to produce the        output signal Y1 n;    -   means for adapting the coefficients of the identification filter        as a function of an adaptation step μ_(n); and    -   means for calculating the adaptation step μ_(n).

This echo canceller is noteworthy in that the adaptation stepcalculation means comprise means for estimating the power P1 n of theinput signal X2 n and the power P3 n of the signal Y3 n and means forcalculating a first coupling variable COR2 characteristic of theacoustic coupling between the input signal X2 n and the signal Y3 n fromthe echo generator system, the adaptation step μ_(n) of theidentification filter being calculated as a function of the estimatedpowers P1 n, P3 n and as a function of the first coupling variable COR2.

Evaluating the above coupling variable COR2 means that the adaptationstep of the filter may be “driven” as a function of the real acousticcoupling between the input signal and the output signal of the echogenerator system. This improves the responsiveness of the echo cancelleras a function of changes in the acoustic environment of the device, andtherefore improves the result of echo processing.

In a preferred embodiment, the adaptation step μ_(n) is obtained fromthe equation:

$\mu_{n} = \frac{P\; 1\; n}{{{\alpha \cdot P}\; 1\; n} + {{COR}\; 2.P\; 3\; n}}$

in which αis a positive constant and P1 n and P3 n are respectively anestimate of the power of the input signal X2 n and an estimate of thepower of the signal Y3 n from the echo generator system, at the timeconcerned.

In one embodiment, the adaptation step calculation means furthercomprise means for calculating a second coupling variable CORcharacteristic of the acoustic coupling between the input signal X2 nfrom the echo generator system and the output signal Y1 n, the secondcoupling variable COR being obtained by calculating the correlationbetween the input signal X2 n and the output signal Y1 n, and theadaptation step μ_(n) of the identification filter being calculated as afunction of the second coupling variable COR.

By additionally taking account of the second coupling variable COR, itis possible to determine the state of convergence of the identificationfilter and thus to apply finer control of the adaptation step.

In a third aspect, the invention provides an echo processing device fora multichannel communications system comprising N receive channels, Nbeing an integer greater than or equal to 2, and M send channels, Mbeing an integer greater than or equal to 1, each of the N receivechannels i comprising an output transducer that produces a soundpressure wave in response to an input signal X2 n(i) derived from adirect signal X1 n(i), each of the M send channels j comprising an inputtransducer that converts a sound pressure wave into an output signal Y1n(j), and said echo processing device being adapted to attenuate, ineach output signal Y1 n(j), echo components stemming from some or all ofthe N input signals X2 n(i) and resulting from the acoustic couplingbetween the input transducer of the send channel concerned and some orall of the M output transducers.

According to an embodiment of the invention the device is noteworthy inthat it comprises:

-   -   means for calculating receive gains Gr_(n)(i) and send gains        Ge_(n)(j);    -   means for applying a receive gain Gr_(n)(i) to each direct        signal X1 n(i) and producing the corresponding input signal X2        n(i):    -   means for applying a send gain Ge_(n)(j) to each output signal        Y1 n(j) and producing the corresponding return signal Y2 n(j);        and    -   means for calculating, for each send channel j, N coupling        variables COR(j,i), for i varying from 1 to N, each of which is        characteristic of the acoustic coupling between the output        signal Y1 n(j) of the send channel and one of the N input        signals X2 n(i);

said gain calculation means being adapted to calculate each receive gainGrn(i) and each send gain Ge_(n)(j) on the basis of the N couplingvariables COR(j,i) calculated for the associated send channel j.

The advantages of this mode of calculating gains in respect of a givenpair of send and receive channels (i, j) are of the same kind as areobtained with a variable gain single-channel device of the invention, asbriefly set out hereinabove.

In a preferred embodiment of the invention, the echo processing devicecomprises means for estimating the instantaneous power P1 n _(i) of eachinput signal X2 n(i) and the instantaneous power P2 n _(j) of eachoutput signal Y1 n(j), said send gain calculation means being adapted tocalculate each send gain Gen(j) on the basis of N variables G(j,i), fori varying from 1 to N, each of which is determined as a function of theestimated power of an input signal X2 n(i) and the estimated power ofthe output signal Y1 n(j) of the send channel concerned and as afunction of the corresponding coupling variable COR(j,i), and each ofthe variables G(j,i) being obtained from the following equation:

${G( {j,i} )} = \frac{P\; 2\; n_{j}}{{P\; 2\; n_{j}} + {{{{COR}( {j,i} )} \cdot P}\; 1\; n_{i}}}$

in which P1 n _(i) and P2 nj are respectively an estimate of the powerof the input signal X2 n(i) concerned and of the power of the outputsignal Y1 n(j) concerned at the time concerned.

In a fourth aspect, the invention provides an echo canceller for amultichannel communications system comprising N receive channels, Nbeing an integer greater than or equal to 2, and M send channels, Mbeing an integer greater than or equal to 1, each of the N receivechannels i comprising an output transducer that produces a soundpressure wave in response to an input signal X2 n(i), and each of the Msend channels j comprising an input transducer that converts a soundpressure wave into an output signal Y1 n(j), the device comprising:

-   -   for each send channel j, N identification filters Fij with        variable coefficients for estimating the acoustic coupling        between each of the N output transducers and the input        transducer of the send channel j, and    -   for each filter Fij, means for adapting the coefficients of the        filter as a function of an adaptation step μ_(n)(i,j) and means        for calculating the adaptation step μ_(n)(i,j).

According to an embodiment of the invention, this device is noteworthyin that it comprises:

-   -   means for estimating the instantaneous power P1 n _(i) of each        input signal X2 n(i) and the instantaneous power P2 n _(j) of        each output signal Y1 n(j), and    -   means for calculating, for each send channel j, N coupling        variables COR(j,i), for i varying from 1 to N, each of which        being characteristic of the acoustic coupling between the output        signal Y1 n(j) of the same channel and one of the N input        signals X2 n(i),    -   the means for calculating the adaptation step μ_(n)(i,j) for a        filter Fij associated with a receive channel i and a send        channel j, being adapted to calculate the adaptation step        μ_(n)(i,j) as a function of the powers P1 n _(i), for i varying        from 1 to N, estimated for the N receive channels, as a function        of the power P2 n _(j) estimated for the send channel j, and as        a function of the N coupling variables COR(j,i), for i varying        from 1 to N, associated with the send channel j.

In a preferred embodiment, an adaptation step μ_(n)(i,j) for a filterFij associated with a receive channel i and a send channel j is obtainedfrom the following equation, in which b_(i) is a positive constant:

${\mu_{n}( {i,j} )} = \frac{P\; 1\; n_{i}}{{{b_{i} \cdot P}\; 1\; n_{i}} + {{{{COR}( {j,i} )} \cdot P}\; 2\; n_{j}} + {\sum\limits_{k \neq i}\;{{{{COR}( {j,k} )} \cdot P}\; 1\; n_{k}}}}$

Further features and advantages of the invention will become apparent inthe course of the following description of preferred embodiments of theinvention, which is given with reference to the appended drawings, inwhich:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a variable gain single-channel echoprocessing device according to a first embodiment of the invention;

FIG. 2 is a block diagram of a single-channel echo processing devicecombining a variable gain system and an echo canceller, according to asecond embodiment of the invention;

FIG. 3 is a block diagram of a single-channel echo canceller accordingto a third embodiment of the invention;

FIG. 4 is a block diagram of a single-channel echo canceller accordingto a fourth embodiment of the invention;

FIG. 5 is a block diagram of a single-channel echo processing device ofthe invention combining the features of the first and fourth embodimentsof the invention;

FIG. 6 is a block diagram of a variable-gain multichannel echoprocessing device according to a fifth embodiment of the invention; and

FIG. 7 is a block diagram of a multichannel echo canceller according toa sixth embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a variable-gain single-channel echo processing deviceaccording to a first embodiment of the invention. This device isintegrated into a hands-free telephone, for example.

As shown in FIG. 1, the device receives and sends digital signals X1 n,Y2 n respectively called the direct signal and the return signal.

The echo processing device comprises a module 36 for calculating thereceive gain (Gr_(n)) and the send gain (Ge_(n)). The receive gainGr_(n) is applied to the direct signal X1 n by a multiplier 10 to obtainan input signal X2 n that is emitted into an echo generator system 26.

Similarly, the send gain Ge_(n) is applied to an output signal Y1 n fromthe echo generator system by a multiplier 12 to produce the returnsignal Y2 n.

The input signal X2 n is delivered to a loudspeaker 22 via adigital-to-analog converter (DAC) 14 and an amplifier 18. The amplifier18 is typically a variable-gain amplifier so that a user of the devicemay adjust the volume of the sound reproduced by the loudspeaker 22 tosuit his convenience.

In a similar manner, the output signal Y1 n is obtained from amicrophone 24 via an amplifier 20 and an analog-to-digital converter(ADC) 16.

In the embodiment shown, the device comprises a single loudspeaker 22and a single microphone 24 forming part of the echo generator system 26.However, the device of the invention shown in FIG. 1 may equally well beapplied to a system in which the input signal X2 n is emitted into theecho generator system by a plurality of loudspeakers 22 reproducing thesame sound signal and the output signal Yin is obtained from the echogenerator system by means of a plurality of microphones 24.

According to the invention, the echo processing device comprises amodule 30 for calculating a coupling variable COR characteristic of theacoustic coupling between the direct signal X1 n or the input signal X2n and the output signal Y1 n.

To this end, the calculation module 30 comprises a calculation unit 34.The coupling variable COR is calculated by the unit 34 and then used bythe gain calculation module 36 to calculate the receive gain Gr_(n) andthe send gain Ge_(n).

In the embodiment shown in FIG. 1, the module 30 for calculating thecoupling variable COR comprises a unit 28 for estimating theinstantaneous power P1 n of the input signal X2 n and/or the directsignal X1 n and an unit for estimating the instantaneous power P2 n ofthe output signal Y1 n.

In this embodiment, the gain calculation module 36 is designed tocalculate the receive gain Gr_(n) and the send gain Ge_(n) on the basisof a variable G calculated by the calculation unit 34 as a functionfirstly of the estimated power P1 n of the direct signal and/or theinput signal and the estimated power P2 n of the output signal, andsecondly as a function of the coupling variable COR.

In a preferred embodiment of the invention, the variable G is determinedby the calculation unit 34 from the following equation:

$\begin{matrix}{G = \frac{P\; 2\; n}{{P\; 2\; n} + {{{COR} \cdot P}\; 1\; n}}} & (1)\end{matrix}$

where P1 n and P2 n are respectively an estimate of the power of thedirect signal X1 n or the input signal X2 n and an estimate of the powerof the output signal Y1 n, at the time concerned.

Accordingly, strong coupling (i.e. a high level of correlation) betweenthe direct signal X1 n or the input signal X2 n and the output signal Y1n yields a low value of the variable G to cancel echo, whereas weakcoupling has the opposite effect on the variable G.

In a preferred embodiment of the invention, the gain calculation means36 determine the receive gain Gr_(n) and the send gain Ge_(n)recursively from the following equations:Ge _(n) =γ·Ge _(n-1)+(1−γ)·GGr _(n)=1−δ·Ge _(n)  (2)

where Ge_(n-1) is the send gain at the preceding calculation time and γand δ are positive constants less than 1.

The above gain calculation equation (2), which is cited by way ofexample only, is derived from a calculation disclosed in French patentNo. 2 748 184, modified in accordance with the invention to take intoaccount the variable G defined above (equation (1)).

In one particular embodiment, good results have been obtained for acalculation at a frequency of 8 kiloHertz (kHz) with γ equal to 0.95.

In the above calculation, the send and receive gains are directly linkedto the variable G, which enables adaptive echo processing as a functionof the real characteristics of the echo generator system. Moreover, therange of variation of the send gain Ge_(n) is a decreasing function ofthe variable G, enabling automatic enhancement, by increasing the gain,of the sound quality as perceived by the remote party if the echocomponent of the signal picked up by the microphone decreases.

Incidentally, it should be noted that the above advantages are obtainedwithout using voice activity detectors and double voice detectors, whichin prior art echo processing devices are complex and sometimesinsufficiently reliable.

Calculation of the Coupling Variable COR

According to the invention, the coupling variable COR whichcharacterizes the acoustic coupling between the direct signal X1 n orthe input signal X2 n and the output signal Y1 n is obtained bycalculating the correlation between the direct signal X1 n or the inputsignal X2 and the output signal Y1 n.

An envelope correlation calculation may be used, for example. Thus inone particular embodiment the coupling variable COR is defined as afunction of the maximum value Maxcor of the values corr(j) of thecorrelation between the direct signal X1 n or the input signal X2 n andthe output signal Y1 n, said correlation values corr(j) being calculatedover a time window considered, and each being obtained from theequation:

$\begin{matrix}{{{corr}(j)} = \frac{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1\;{(i) \cdot P}\; 2\;( {i + j} )}}{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1^{2}(i)}}} & (3)\end{matrix}$

in which i is a sampling time in the calculation time window of durationLM, j is a shift value between the input signal X2 n and the outputsignal Y1 n, and P1(t) and P2(t) are respectively an estimate of thepower of the direct signal X1 n or the input signal X2 n and an estimateof the power of the output signal Y1 n, at a time t.

In practice, the envelope correlation calculation is effected over timewindows of 1 second duration for each signal (input and output) and witha maximum time shift of 300 milliseconds between the signals. Thecalculation is effected at a reduced sampling frequency of 125 Hertz.

In this embodiment, very good results are obtained with the variable CORdefined by the following equation, in which Exp designates theexponential function and k is a positive constant:COR=Exp(k.Maxcor)   (4)

In practice, very good results are obtained with k equal to 3. Limitingthe term Exp(3.Maxcor) to 25, corresponding to a maximum correlation of1.07, is recommended.

A single-channel echo processing device according to a second embodimentof the invention is described next with reference to FIG. 2. This devicecombines a variable gain system like that described hereinabove withreference to FIG. 1 and an echo canceller.

The echo processing device represented in FIG. 2 comprises, like thatrepresented in FIG. 1, a module 36 for calculating the receive gain(Gr_(n)) and the send gain (Ge_(n)) and a module 30 for determining thevariable COR to evaluate the acoustic coupling between the direct signalX1 n or the input signal X2 n and the output signal Y1 n. The featuresand operation of the FIG. 2 modules 30 and 36 are identical to those ofthe FIG. 1 modules.

According to the invention, the device may further include an echocanceller 40 receiving at its input the input signal X2 n emitted intothe echo generator system 26 and a signal Y3 n from the echo generatorsystem 26. The echo canceller 40 conventionally comprises a finiteimpulse response identification filter 42 whose response isrepresentative of the response of the echo generator system 26.

In operation, the identification filter 42 produces a filtering signalSn and subtracts the filtering signal Sn from the signal Y3 n by meansof a subtractor 44. It then produces the output signal Y1 n that isreceived as input by the multiplier 12, in order to apply to it a sendgain Ge_(n) calculated by the module 36.

In this embodiment, the system is initialized with the echo canceller 40inactive (the identification filter 42 has not yet converged) toguarantee stability with no Larsen effect. Then, when the filter hasconverged, the coupling variable COR is evaluated non-intrusively by themodule 30. In this embodiment the correlation referred to is thatbetween the direct signal X1 n or the input signal X2 n and the signalY1 n that constitutes the “residual” signal from the echo canceller 40.The acoustic coupling is then evaluated cyclically to adapt the send andreceive gains automatically as a function of acoustic couplingvariations.

In this embodiment, the effects of a conventional echo canceller 40 andthose of a variable gain echo processing device of the invention(FIG. 1) are combined to optimize echo processing.

In practice, in this embodiment, very good results are obtained with thevariable COR defined as follows, as a function of Maxcor (see above):COR=0.75·Exp(Maxcor)   (5)

A single-channel echo canceller according to a third embodiment of theinvention is described next with reference to FIG. 3. In thisembodiment, the principle of estimating the acoustic coupling betweenthe input and output signals of an echo generator system, includingcalculation of the coupling variable COR as described hereinabove, isapplied to calculating the adaptation step of the filter of an echocanceller.

As shown in FIG. 3, an echo canceller of the invention conventionallycomprises a finite impulse response identification filter 42 whoseresponse is representative of the response of the echo generator system26. The echo generator system comprises the combination of theloudspeaker 22, the microphone 24 and their physical environment (walls,background noise, etc.).

The filter 42 receives at its input an input signal X2 n that is emittedinto the echo generator system 26 via a DAC 14 and an amplifier 18, andgenerates a filtering signal Sn.

The echo canceller comprises a subtractor 44 that receives a signal Y3 nfrom the echo generator system at its input via an amplifier 20 and anADC 16. At least one component of the signal Y3 n is therefore aresponse of the echo generator system to the input signal X2 n.

Furthermore, the subtractor 44 receives the filtering signal Sn at itsinput and therefore subtracts the filtering signal Sn from the signal Y3n to produce an output signal Y1 n.

The echo canceller comprises a module 46 for updating the coefficientsof the identification filter as a function of an adaptation step μ_(n).It finally comprises a calculation module 50 for calculating theadaptation step μ_(n).

The module 50 for calculating the adaptation step of the filtercomprises units 28, 48 for estimating the power P1 n of the input signalX2 n and the power P3 n of the signal Y3 n.

The module 50 further comprises a unit 52 for calculating a couplingvariable COR2 characteristic of the acoustic coupling between the inputsignal X2 n and the signal Y3 n coming from the echo generator system26.

The module 50 finally comprises a unit 54 for calculating the adaptationstep. According to the present invention, the adaptation step μ_(n) ofthe identification filter is calculated as a function of the estimatedpowers P1 n, P3 n and the coupling variable COR2.

In a preferred embodiment of the invention, the adaptation step μ_(n) isobtained from the following equation:

$\begin{matrix}{\mu_{n} = \frac{P\; 1\; n}{{{\alpha \cdot P}\; 1\; n} + {{COR}\; 2.P\; 3\; n}}} & (6)\end{matrix}$

in which αis a positive constant and P1 n and P3 n are respectively anestimate of the power of the input signal X2 n and an estimate of thepower of the signal Y3 n from the echo generator system, at the timeconcerned.

Evaluating the above coupling variable COR2 therefore enables theadaptation step of the filter to be “driven” as a function of the realacoustic coupling between the input signal and the output signal of theecho generator system. This improves the responsiveness of the echocanceller as a function of changes in the acoustic environment of thedevice—for example after a variation in the sound reproduction volume bythe user of the device or use of the device in a noisy environment(street, car, etc.)—and therefore improves the result of echoprocessing.

According to the same principle as applies to the variable COR definedabove in relation to FIG. 1, the coupling variable COR2 is obtained bycalculating the correlation between the input signal X2 n and the signalY3 n. In practice this is also an envelope correlation calculation. In apreferred embodiment, the coupling variable COR2 is defined as being afunction of the maximum value Maxcor2 of the correlation values corr2(j)calculated over a time window. Each of the correlation values corr2(j)is calculated from the following equation:

$\begin{matrix}{{{corr2}(j)} = \frac{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1\;{(i) \cdot P}\; 3\;( {i + j} )}}{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1^{2}(i)}}} & (7)\end{matrix}$

in which:

i is a sampling time in the calculation time window of duration LM and jis a shift value between the input signal X2 n and the signal Y3 n; and

P1(t) and P3(t) are respectively an estimate of the power of the inputsignal X2 n and an estimate of the power of the signal Y3 n, at the timet concerned.

In this embodiment, very good results have been obtained with thevariable COR2 defined by the following equation, in which k is apositive constant:

$\begin{matrix}{{{COR}\; 2}\; = \frac{k}{{Maxcor}\; 2}} & (8)\end{matrix}$

In a fourth embodiment of the invention, the single-channel echocanceller described hereinabove has added to it a module for calculatinga second coupling variable COR, so named by analogy with that from FIG.1, that is characteristic of the acoustic coupling between the inputsignal X2 n of the echo generator system and the output signal Y1 ncoming from the subtractor 44 of the echo canceller.

FIG. 4 shows an echo canceller according to this fourth embodiment. Asshown in FIG. 4, the echo canceller comprises a module 50 forcalculating the adaptation step μ_(n) similar to that described withreference to FIG. 3. The device further comprises a unit 30a forcalculating a second coupling variable COR.

The variable COR is characteristic of the acoustic coupling between theinput signal X2 n of the echo generator system 26 and the output signalY1 n. The second coupling variable COR is obtained by calculating thecorrelation between the input signal X2 n and the output signal Y1 n.

The calculation unit 30 a is similar to the unit 30 described above withreference to FIG. 1.

In the embodiment shown in FIG. 4, the second variable COR is obtainedby the same basic process as the variable COR defined above withreference to FIG. 1, i.e. by means of an envelope correlationcalculation applied to the input signal X2 n and the output signal Y1 n.In particular, the variable COR is defined as being a function of themaximum value Maxcor of the values of the correlation corr(j) betweenthe input signal X2 n and the output signal Y1 n.

The second coupling variable COR calculated by the unit 30 a is suppliedto the unit 54 for calculating the adaptation step μ_(n) of the filter(see FIG. 3), with the result that this step is also calculated as afunction of the second coupling variable COR.

In practice, the adaptation step μ_(n) is calculated from the followingequation:

$\begin{matrix}{\mu_{n} = {\frac{COR}{{COR}\; 2} \cdot \frac{P\; 1\; n}{{{\alpha.P}\; 1\; n} + {{COR}\;{2 \cdot P}\; 3\; n}}}} & (9)\end{matrix}$

in which αis a positive constant and P1 n and P3 n are respectively anestimate of the power of the input signal X2 n and an estimate of thepower of the signal Y3 n from the echo generator system, at the timeconcerned.

In the embodiment in which the variable COR is a predetermined functionf of the variable Maxcor and the variable COR2 is a predeterminedfunction g of the variable Maxcor2 (see above), the above equation (9)may be expressed in the following form:

$\begin{matrix}{\mu_{n} = {\frac{f({Maxcor})}{g( {{Maxcor}\; 2} )} \cdot \frac{P\; 1\; n}{{{\alpha\;.P}\; 1\; n} + {{COR}\;{2 \cdot P}\; 3\; n}}}} & ( {9a} )\end{matrix}$

By additionally taking into account the second coupling variable COR, itis possible to determine the state of convergence of the identificationfilter and thus to achieve finer control of the adaptation step.

Another embodiment of the invention combines the echo processing devicedescribed above with reference to FIG. 1 and the device described abovewith reference to FIG. 4. FIG. 5 shows a device of this kind.

In FIG. 5, the items referenced 10, 12, 36, 30 are identical to thoserepresented in FIG. 1 and constitute a variable gain single-channel echoprocessing device of the invention. Furthermore, the items 50, 46, 40are identical to those of the echo canceller shown in FIG. 4. When theunits 30 and 50 are adapted so that the unit 30 is able to supply thevariable COR to the unit 50 and the unit 50 is able to calculate theadaptation step of the filter 42 as a function of the variables COR,COR2, as explained above, then a combination of the systems describedwith reference to FIGS. 1 and 4 is obtained that combines the advantagesof each of those two systems.

The present invention also applies to echo processing devices intendedfor a multichannel communications system.

A variable gain multichannel echo processing device constituting a fifthembodiment of the invention is described next with reference to FIG. 6.

As shown in FIG. 6, a variable gain multichannel echo processing deviceof the invention is intended to be used in a multichannel communicationssystem comprising N receive channels, where N is an integer greater thanor equal to 2, and M send channels, where M is an integer greater thanor equal to 1.

Each of the N receive channels i comprises an output transducer LSi,typically a loudspeaker, which produces a sound pressure wave inresponse to an input signal X2 n(i) derived from a direct signal X1n(i).

Each of the M send channels j comprises an input transducer MCj,typically a microphone, which converts a sound pressure wave into anoutput signal Y1 n(j).

An echo processing device of the above kind is intended to attenuate ineach output signal Y1 n(j) echo components stemming from some or all ofthe N input signals X2 n(i) and resulting from acoustic coupling betweenthe microphone of the send channel concerned and some or all of the Nloudspeakers.

As shown in FIG. 6, a variable gain multichannel echo processing deviceof the invention comprises a module 64 for calculating receive gainsGr_(n)(i) and send gains Ge_(n)(j).

It further comprises N multipliers 68 adapted to apply a receive gainGr_(n)(i) to each direct signal X1 n(i) and produce the correspondinginput signal X2 n(i).

Similarly, the device comprises multipliers 66 adapted to apply a sendgain Ge_(n)(j) to each output signal Y1 n(j) and produce a correspondingreturn signal Y2 n(j).

It further comprises a module 62 for calculating N coupling variablesCOR(j,i), for i varying from 1 to N, for each send channel j, each ofthe N variables being characteristic of the acoustic coupling betweenthe output signal Y1 n(j) of the send channel j concerned and one of theN input signals X2 n(i).

According to the invention, the gain calculation module 64 calculateseach receive gain Gr_(n)(i) and each send gain Ge_(n)(j) on the basis ofthe N coupling variables COR(j,i) calculated for the associated sendchannel j.

The advantages relating to this gain calculation method with respect toa given pair (i,j) of receive and send channels are of the same natureas those obtained with the variable gain single-channel device of theinvention described above with reference to FIG. 1.

Furthermore, a preferred embodiment of the multichannel echo processingdevice shown in FIG. 6 comprises a power calculation module (not shown)adapted to estimate the instantaneous power P1 n _(i) of each inputsignal X2 n(i) and the instantaneous power P2 n _(j) of each outputsignal Y1 n(j).

In this embodiment, the correlation variable COR calculation module 62also calculates N variables G(j,i) for i varying from 1 to N, each ofwhich is determined as a function of the estimated power P1 n of aninput signal X2 n(i) and the estimated power P2 n _(j) of the outputsignal Y1 n(j) of the send channel concerned. According to theinvention, each of the variables G(j,i) is obtained from the followingequation:

$\begin{matrix}{{G( {j,i} )} = \frac{P\; 2\; n_{j}}{{P\; 2\; n_{j}} + {{{{COR}( {j,i} )} \cdot P}\; 1\; n_{i}}}} & (10)\end{matrix}$

in which P1 n _(i) and P2 n _(j) are respectively an estimate of thepower of the input signal X2 n(i) concerned and an estimate of the powerof the output signal Y1 n(j) concerned, at the time concerned.

The gain calculation module 64 then calculates each send gain Ge_(n)(j)on the basis of the N variables G(j,i) as a function of thecorresponding coupling variable COR(j,i).

In a preferred embodiment, each send gain Ge_(n)(j) is determined fromthe minimum value of the N variables G(j,i), for i varying from 1 to N,calculated for the associated send channel j.

In practice, each send gain Ge_(n)(j) is determined from the followingequation:Ge _(n)(j)=γ·Ge_(n-1)(j)+(1−γ)·min_(i)(G(j,i))   (11)

in which Ge_(n-1)(j) is the send gain of the send channel j at the timeof the preceding calculation, γ is a positive constant less than 1, andmin_(i)(G(j,i)) is the minimum value of the N variables G(j,i) for ivarying from 1 to N.

Taking the minimum value min_(i)(G(j,i)), the lowest gain (i.e. thehighest attenuation) is applied to the channel j concerned, this gaintherefore taking into account the greatest coupling value on allpossible echo paths of the system.

Preferably (although this is not mandatory), in combination with themethod of calculating the send gain explained hereinabove, all thereceive gains Gr_(n)(i) have the same value, determined from thefollowing equation:Gr _(n)(i)=1−δ·max_(j)(Ge _(n)(j))   (12)

in which δ is a positive constant less than 1 and max_(j)(Ge_(n)(j)) isthe maximum value of the M send gains Ge_(n)(j), for j varying from 1 toM.

However, in a different embodiment of the device, shown in FIG. 6, eachreceive gain Gr_(n)(i) is made equal to 1. This solution has theadvantage of simplifying the calculation of the gains, combined withvery good echo processing results.

Calculation of Each Coupling Variable COR(j,i)

According to the invention, each coupling variable COR(j,i) is obtainedfrom a calculation of the correlation between the corresponding outputsignal Y1 n(j) and input signal X2 n(i). In a preferred embodiment, thecalculation is an envelope correlation calculation.

In practice, each coupling variable COR(j,i) is obtained from themaximum value Maxcor of the values corr_(ji)(d) of the correlationbetween the corresponding output signal Y1 n(j) and input signal X2n(i), these correlation values corr_(ji)(d) being calculated over apredefined time window. Each of the correlation values is obtained fromthe following equation:

$\begin{matrix}{{{corr}_{ji}(d)} = \frac{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{{n_{i}(c)} \cdot P}\; 2\;{n_{j}( {c + d} )}}}{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{n_{i}^{2}(c)}}}} & (13)\end{matrix}$

in which c is a sampling time in the calculation time window of durationLM, d is a shift value between the input signal X2 n(i) and the outputsignal Y1 n(j), and P1 n _(i)(t) and P2 n _(j)(t) are respectively anestimate of the power of the input signal X2 n(i) and an estimate of thepower of the output signal Y1 n(j) at a time t.

A multichannel echo canceller constituting a sixth embodiment of theinvention is described next with reference to FIG. 7. This embodimentmay be considered as a generalization to the multichannel situation ofthe single-channel echo cancellers described above with reference toFIGS. 3 and 4.

As shown in FIG. 7, a multichannel echo canceller of the inventioncomprises N receive channels, where N is an integer greater than orequal to 2, and M send channels, where M is an integer greater than orequal to 1.

Each of the N receive channels i comprises an output transducer(loudspeaker) LSi that produces a sound pressure wave in response to aninput signal X2 n(i). Each of the M send channels j comprises an inputtransducer (microphone) MCj that converts a sound pressure wave into anoutput signal Y1 n(j).

Furthermore, the echo canceller comprises, for each send channel j, Nidentification filters Fij with variable coefficients for estimating theacoustic coupling between each of the N loudspeakers LSi and themicrophone MCj of the send channel j. It further comprises, for eachfilter Fij, means (not shown) for adapting the coefficients of thefilter as a function of an adaptation step μ_(n)(i,j) and means (notshown) for calculating the adaptation step μ_(n)(i,j).

Each filter Fij associated with a receive channel i and a send channel jgenerates a filtering signal that is subtracted from the output signalY1 n(j) to produce a filtered signal Y2 n(j).

According to the invention, the device further comprises means (notshown) for estimating the instantaneous power P1 n _(i) of each inputsignal X2 n(i) and the instantaneous power P2 n _(j) of each outputsignal Y1 n(j).

It also comprises means (not shown) for calculating, for each sendchannel j, N coupling variables COR(j,i), for i varying from 1 to N,each of which being characteristic of the acoustic coupling between theoutput signal Y1 n(j) of the send channel concerned and one of the Ninput signals X2 n(i).

The means for calculating the adaptation step μ_(n)(i,j) for a filterFij associated with a given receive channel i and a given send channel jcalculate the adaptation step μ_(n)(i,j) as a function of:

the estimated powers P1 n _(i) (for i varying from 1 to N) calculatedfor the N receive channels i,

the estimated power P2 n _(j) calculated for the send channel j, and

the N coupling variables COR(j,i), for i varying from 1 to N, associatedwith the send channel j concerned.

Calculation of Each Coupling Variable COR(j,i)

In this embodiment, each coupling variable COR(j,i) is obtained from acorrelation calculation between the output signal Y1 n(j) and the inputsignal X2 n(i) associated with the pair of receive and send channels(i,j) concerned.

As in the other embodiments of the invention described above, in apreferred embodiment, the correlation calculation is an envelopecorrelation calculation.

In practice, each coupling variable COR(j,i) is obtained from themaximum value Maxcor(j,i) of the values corr_(ji)(d) of the correlationcalculated over a respective predefined time window, each of thecorrelation values corr_(ji)(d) being calculated from the followingequation:

$\begin{matrix}{{{corr}_{ji}(d)} = \frac{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{{n_{i}(c)} \cdot P}\; 2\;{n_{j}( {c + d} )}}}{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{n_{i}^{2}(c)}}}} & (14)\end{matrix}$

in which c is a sampling time in the calculation time window of durationLM, d is a shift value between the input signal X2 n(i) and the outputsignal Y1 n(j), and P1 n _(i)(t) and P2 n _(j)(t) are respectively anestimate of the power of the input signal X2 n(i) and an estimate of thepower of the output signal Y1 n(j) at a time t.

In practice, each coupling variable COR(j,i) is related to the maximumvalue Maxcor(j,i) of the correlation values corr_(ji)(d) by thefollowing equation, in which k is a positive constant:

$\begin{matrix}{{{COR}( {j,i} )} = \frac{k}{{Maxcor}( {j,i} )}} & (15)\end{matrix}$Calculation of the Adaptation Step μ_(n)(i,j) for a Filter Fij

In this embodiment, an adaptation step μ_(n)(i,j) for a filter Fijassociated with a receive channel i and a send channel j is obtainedfrom the following equation, in which b_(i) is a positive constant:

$\begin{matrix}{{\mu_{n}( {i,j} )} = \frac{P\; 1\; n_{i}}{{{b_{i} \cdot P}\; 1\; n_{i}} + {{{{COR}( {j,i} )} \cdot P}\; 2\; n_{j}} + {\sum\limits_{k \neq i}\;{{{{COR}( {j,k} )} \cdot P}\; 1\; n_{k}}}}} & (16)\end{matrix}$

Thanks to the presence in the above expression of the term

$\sum\limits_{k \neq i}\;{{{{COR}( {j,k} )} \cdot P}\; 1\; n_{k}}$for the step μ_(n)(i,j), the receive channels other than the channel iconcerned do not interfere with the convergence of the filter Fij, andthis is achieved in conjunction with automatic reduction of the value ofthe step. Furthermore, the presence of the variables COR(j,k) providesan indication of the real influence on the send channel j concerned ofreceive channels other than the channel i concerned.

In a similar manner to the single-channel situation described above withreference to FIG. 4, an embodiment of the multichannel echo canceller,shown in FIG. 7, may further comprise means for calculating, for eachsend channel j, N second coupling variables COR2(j,i) for i varying from1 to N.

Each of the second coupling variables is characteristic of the acousticcoupling between the filtered signal Y2 n(j) of the send channel jconcerned and one of the N input signals X2 n(i).

In this embodiment, the adaptation step μ_(n)(i,j) of an identificationfilter Fij associated with a receive channel i and a send channel j iscalculated as a function of the first N coupling variables COR(j,i) andthe second N coupling variables COR2(j,i).

In a preferred embodiment, the adaptation step μ_(n)(i,j) for a filterFij associated with a receive channel i and a send channel j is obtainedfrom the following equation, in which b_(i) is a positive constant:

$\begin{matrix}{{\mu_{n}( {i,j} )} = {\frac{{COR}( {j,i} )}{{COR}\; 2( {j,i} )} \cdot \frac{P\; 1\; n_{i}}{{{b_{i} \cdot P}\; 1\; n_{i}} + {{{{COR}( {j,i} )} \cdot P}\; 2\; n_{j}} + {\sum\limits_{k \neq i}\;{{{{COR}( {j,k} )} \cdot P}\; 1\; n_{k}}}}}} & (17)\end{matrix}$

A variable gain multichannel echo processing device of the invention(FIG. 6) may be combined with a multichannel echo canceller of theinvention (FIG. 7) to combine their advantages.

In this case, this kind of multichannel device (not shown in thedrawings) comprises, for each pair comprising a receive channel i and asend channel j, gain application means adapted to apply a receive gainGr_(n)(i) to the input signal X2 n(i) and a send gain Ge_(n)(j) to thefiltered signal Y2 n(j).

The gains Gr_(n)(i), Ge_(n)(j) are then calculated on the basis of the Nsecond coupling variables COR2(j,i) determined for the send channel j,using the same basic principle as the device described above withreference to FIG. 6.

In practice, the various echo processing devices of the presentinvention described hereinabove may be obtained in the usual way byprogramming a digital signal processor (DSP). They may also beimplemented by means of application-specific integrated circuits (ASIC).

Of course, the present invention is in no way limited to the embodimentsdescribed here, and to the contrary encompasses any variant that will beevident to the person skilled in the art.

1. An echo processing device for attenuating echo components of a directsignal X1 n in a return signal Y2 n, said device comprising: means forcalculating a receive gain Gr_(n) and a send gain Ge_(n); first gainapplication means for applying the receive gain Gr_(n) to the directsignal and producing an input signal X2 n emitted into an echo generatorsystem; second gain application means for applying the send gain Ge_(n)to an output signal Y1 n from the echo generator system and producingthe return signal Y2 n; and means for obtaining a coupling variable CORcharacteristic of the acoustic coupling between the direct signal X1 nor the input signal X2 n and the output signal Y1 n, by calculating acorrelation between the direct signal X1 n or the input signal X2 n andthe output signal Y1 n; wherein said gain calculation means isconfigured to calculate the receive gain Gr_(n) and the send gain Ge_(n)based on said coupling variable.
 2. An echo processing device accordingto claim 1, comprising means for estimating the instantaneous power ofthe direct signal X1 n or the input signal X2 n and the instantaneouspower of the output signal Y1 n, said gain calculation means beingadapted to calculate the receive gain Gr_(n) and the send gain Ge_(n) onthe basis of a variable G determined as a function of the estimatedpower of the direct signal or the input signal and the estimated powerof the output signal, and as a function of the coupling variable COR, inaccordance with the following equation:$G = \frac{P\; 2\; n}{{P\; 2\; n} + {{{COR} \cdot P}\; 1\; n}}$ where P1n and P2 n are respectively an estimate at the time concerned of thepower of the direct signal X1 n or the input signal X2 n and the powerof the output signal Y1 n.
 3. An echo processing device according toclaim 2, in which the gain calculation means determine the receive gainGr_(n) and the send gain Ge_(n) recursively from the followingequations:Ge _(n) =γ·Ge _(n-1)+(1−γ)·GGr _(n)=1−δ·Ge _(n) where Ge_(n-1) is the send gain at the precedingcalculation time and γand δare positive constants less than
 1. 4. Anecho processing device according to claim 1, in which the calculation ofthe correlation between the direct signal X1 n or the input signal X2 nand the output signal Y1 n is an envelope correlation calculation.
 5. Anecho processing device according to claim 4, in which, in said envelopecorrelation calculation, the coupling variable COR is a function of themaximum value Maxcor of the values corr(j) of the correlation betweenthe direct signal X1 n or the input signal X2 n and the output signal Y1n, said correlation values corr(j) being calculated over a time windowconsidered, and each being obtained from the equation:${{corr}(j)} = \frac{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1\;{(i) \cdot P}\; 2\;( {i + j} )}}{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1^{2}(i)}}$in which i is a sampling time in the calculation time window of durationLM, j is a shift value between the input signal X2 n and the outputsignal Y1 n , and P1(j) and P2(j) are respectively an estimate of thepower of the direct signal X1 n or the input signal X2 n and an estimateof the power of the output signal Y1 n at a time t.
 6. An echoprocessing device according to claim 5, in which the coupling variableCOR is linked to the maximum value Maxcor of the correlation valuescorr(j) calculated over a calculation time window considered from theequation:COR=Exp(k.Maxcor) in which Exp is the exponential function and k is apositive constant.
 7. An echo processing device according to claim 1, inwhich the input signal X2 n is emitted into the echo generator system byat least one loudspeaker and the output signal Y1 n is obtained from theecho generator system by at least one microphone.
 8. An echo processingdevice according to claim 1, further comprising an echo cancellerreceiving at its input said input signal X2 n emitted into the echogenerator system and a signal Y3 n from the echo generator system, theecho canceller comprising a finite impulse response identificationfilter whose response is representative of the response of the echogenerator system, and the identification filter being adapted togenerate a filtering signal Sn and comprising means for subtracting thefiltering signal Sn from the signal Y3 n to produce the output signal Y1n that is received at the input of said send gain application means. 9.An echo canceller for attenuating in an output signal Y1 n echocomponents of an input signal X2 n emitted into an echo generatorsystem, said device comprising: a finite impulse response identificationfilter whose response is representative of the response of the echogenerator system, receiving the input signal X2 n at its input andgenerating a filtering signal Sn; subtraction means receiving at aninput a signal Y3 n from the echo generator system, at least onecomponent of which is a response of the echo generator system to theinput signal X2 n, and the filtering signal Sn, and adapted to subtractthe filtering signal Sn from the signal Y3 n and to produce the outputsignal Y1 n; means for adapting the coefficients of the identificationfilter as a function of an adaptation μ_(n); and means for calculatingthe adaptation μ_(n), said adaptation calculation means comprising meansfor estimating the power P1 n of the input signal X2 n and the power P3n of the signal Y3 n and means for obtaining a first coupling variableCOR2 characteristic of the acoustic coupling between the input signal X2n and the signal Y3 n from the echo generator system, by calculating acorrelation between the input signal X2 n and the signal Y3 n; whereinthe adaptation μ_(n) of the identification filter is calculated as afunction of the estimated powers P1 n, P3 n and as a function of thefirst coupling variable COR2.
 10. A device according to claim 9, inwhich the adaptation μ_(n) is obtained from the equation:$\mu_{n} = \frac{P\; 1\; n}{{{\alpha \cdot P}\; 1\; n} + {{COR}\; 2.P\; 3\; n}}$in which αis a positive constant and P1 n and P3 n are respectively anestimate of the power of the input signal X2 n and an estimate of thepower of the signal Y3 n from the echo generator system at the timeconcerned.
 11. A device according to claim 9, in which the calculationof the correlation between the input signal X2 n and the signal Y3 n isan envelope correlation calculation.
 12. A device according to claim 11,in which the first coupling variable COR2 is a function of the maximumvalue Maxcor2 of correlation values corr2(j) calculated over a timewindow considered, each of the correlation values corr2(j) beingcalculated from the following equation:${{corr2}(j)} = \frac{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1{(i) \cdot P}\; 3( {i + j} )}}{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1^{2}(i)}}$in which: i is a sampling time in the calculation time window ofduration LM and j is a shift value between the input signal X2 n and thesignal Y3 n; and P1(t) and P3(t) are respectively an estimate of thepower of the input signal X2 n and an estimate of the power of thesignal Y3 n at the time t concerned.
 13. A device according to claim 12,in which the first coupling variable COR2 is linked to the maximum valueMaxcor2 of said correlation values corr2(j) by the following equation,in which k is a positive constant:${{COR}\; 2} = {\frac{k}{{Maxcor}\; 2}.}$
 14. An echo cancelleraccording to claim 9, in which the adaptation step calculation meansfurther comprise means for calculating a second coupling variable CORcharacteristic of the acoustic coupling between the input signal X2 nfrom the echo generator system and the output signal Y1 n, the secondcoupling variable COR being obtained by calculating the correlationbetween the input signal X2 n and the output signal Y1 n, and theadaptation step μ_(n) of the identification filter being calculated asalso a function of the second coupling variable COR.
 15. An echocanceller according to claim 14, in which the second coupling variableCOR is obtained from an envelope correlation calculation between theinput signal X2 n and the output signal Y1 n.
 16. An echo cancelleraccording to claim 15, in which the second coupling variable COR is afunction of the maximum value Maxcor of the values corr(j) of thecorrelation between the input signal X2 n and the output signal Y1 n,said correlation values corr(j) being calculated over a time windowconsidered and each of them being obtained from the equation:${{corr}(j)} = \frac{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1{(i) \cdot P}\; 2( {i + j} )}}{\sum\limits_{i = 0}^{{LM} - 1}\;{P\; 1^{2}(i)}}$in which i is a sampling time in the calculation window of duration LM,j is a shift value between the input signal X2 n and the output signalY1 n, and P1(t) and P2(t) are respectively an estimate of the power ofthe input signal X2 n and an estimate of the power of the output signalY1 n at a time t.
 17. An echo canceller according to claim 14, in whichthe adaptation step μ_(n) is calculated from the equation:$\mu_{n} = {\frac{COR}{{COR}\; 2} \cdot \frac{P\; 1\; n}{{{\alpha.P}\; 1\; n} + {{COR}\;{2 \cdot P}\; 3\; n}}}$in which αis a positive constant and P1 n and P3 n are respectively anestimate of the power of the input signal X2 n and an estimate of thepower of the signal Y3 n from the echo generator system at the timeconcerned.
 18. An echo processing device for a multichannelcommunications system comprising N receive channels, N being an integergreater than or equal to 2, and M send channels, M being an integergreater than or equal to 1, each of the N receive channels i comprisingan output transducer (LSi) that produces a sound pressure wave inresponse to an input signal X2 n(i) derived from a direct signal X1n(i), each of the M send channels j comprising an input transducer (MCj)that converts a sound pressure wave into an output signal Y1 n(j), saidecho processing device being adapted to attenuate in each output signalY1 n(j) echo components stemming from some or all of the N input signalsX2 n(i) and resulting from the acoustic coupling between the inputtransducer of the send channel concerned and some or all of the M outputtransducers, said device comprising: means for calculating receive gainsGr_(n)(i) and send gains Ge_(n)(j); means for applying receive gainsGr_(n)(i) to each direct signal X1 n(i) and producing a correspondinginput signal X2 n(i); means for applying send gains Ge_(n)(j) to eachoutput signal Y1 n(j) and producing the corresponding return signal Y2n(j); and means for calculating, for each send channel j, N couplingvariables COR(j,i), for i varying from 1 to N, each of which beingcharacteristic of the acoustic coupling between the output signal Y1n(j) of the send channel and one of the N input signals X2 n(i), eachcoupling variable COR(j,i) being obtained by calculating a correlationbetween the corresponding output signal Y1 n(j) and the correspondinginput signal Y2 n(i); wherein said gain calculation means is configuredto calculate each receive gain Gr_(n)(i) and each send gain Ge_(n)(j)based on the N coupling variables COR(J,i) calculated for the associatedsend channel j.
 19. A device according to claim 18, comprising means forestimating the instantaneous power P1 n _(i), of each input signal X2n(i) and the instantaneous power P2 n _(j) of each output signal Y1n(j), said send gain calculation means being adapted to calculate eachsend gain Ge_(n)(j) on the basis of N variables G(j,i), for i varyingfrom 1 to N, each of which is determined as a function of the estimatedpower of an input signal X2 n(i) and the estimated power of the outputsignal Y1 n(j) of the send channel concerned and as a function of thecorresponding coupling variable COR(j,i), each of the variables G(j,i)being obtained from the following equation:${G( {j,i} )} = \frac{P\; 2\; n_{j}}{{P\; 2\; n_{j}} + {{{{COR}( {j,i} )} \cdot P}\; 1\; n_{i}}}$in which P1 n _(i) and P2 n _(j) are respectively an estimate of thepower of the input signal X2 n(i) concerned and an estimate of the powerof the output signal Y1 n(j) concerned at the time concerned.
 20. Adevice according to claim 19, in which each send gain Ge_(n)(j) isdetermined from the minimum value of the N variables G(j,i), for ivarying from 1 to N, calculated for the associated send channel j.
 21. Adevice according to claim 20, in which each send gain Ge_(n)(j) isdetermined from the equation:Ge _(n)(j)=γ·Ge _(n-1)(j)+(1−γ)·min_(i)(G(j,i)) in which Ge_(n-1)(j) isthe send gain of the send channel j at the time of the precedingcalculation, γ is a positive constant less than 1, and min_(i) (G(j,i))is the minimum value of the N variables G(j,i) for i varying from 1 toN.
 22. A device according to claim 21, in which all the receive gainsGr_(n)(i) have the same value, which is determined from the equation:Gr _(n)(i)=1−δ·max_(j)(Ge _(n)(j)) in which γ is a positive constantless than 1 and max_(j)(Ge_(n)(j)) is the maximum value of the M sendgains Ge_(n)(j), for j varying from 1 to M.
 23. A device according toclaim 18, in which each of said receive gains Gr_(n)(i) is equal to 1.24. A device according to claim 18, in which the calculation of thecorrelation between an output signal Y1 n(j) and an input signal X2 n(i)is an envelope correlation calculation.
 25. A device according to claim24, in which, in said envelope correlation calculation, each couplingvariable COR(j,i) is a function of the maximum value Maxcor of thevalues corr_(ji)(d) of the correlation between the output signal Y1 n(j)and the input signal X2 n(i), said correlation values corr_(ji)(d) beingcalculated over a predefined time window and each obtained from theequation:${{corr}_{ji}(d)} = \frac{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{{n_{i}(c)} \cdot P}\; 2\;{n_{j}( {c + d} )}}}{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{n_{i}^{2}(c)}}}$in which c is a sampling time in the calculation time window of durationLM d is a shift value between the input signal X2 n(i) and the outputsignal Y1 n(j), and P1 n _(i)(t) and P2 n _(j)(t) are respectively anestimate of the power of the input signal X2 n(i) and an estimate of thepower of the output signal Y1 n(j) at a time t.
 26. An echo cancellerfor a multichannel communications system comprising N receive channels,N being an integer greater than or equal to 2, and M send channels, Mbeing an integer greater than or equal to 1, each of the N receivechannels i comprising an output transducer (LSi) that produces a soundpressure wave in response to an input signal X2 n(i), and each of the Msend channels j comprising an input transducer (MCj) that converts asound pressure wave into an output signal Y1 n(j), the echo cancellercomprising: for each send channel j, N identification filters Fij withvariable coefficients for estimating the acoustic coupling between eachof the N output transducers (LSi) and the input transducer (MCj) of thesend channel j, and for each filter Fij, means for adapting thecoefficients of the filter as a function of an adaptation stepμ_(n)(i,j) and means for calculating the adaptation step μ_(n)(i,j),means for estimating the instantaneous power P1 n _(i) of each inputsignal X2 n(i) and the instantaneous power P2 nj of each output signalY1 n(j), and means for calculating, for each send channel j, N couplingvariables COR(j,i), for i varying from 1 to N, each of which beingcharacteristic of the acoustic coupling between the output signal Y1n(j) of the send channel j and one of the N input signals X2 n (i), eachcoupling variable COR(j,i) being obtained by calculating a correlationbetween the output signal Y1 n(j) and the input signal X2 n(i); whereinthe means for calculating the adaptation step μ_(n)(i,j) for a filterFij associated with a receive channel i and a send channel j isconfigured to calculate the adaptation step μ_(n)(i,j) as a function ofthe powers P1 n _(i), for i varying from 1 to N, estimated for the Nreceive channels, as a function of the estimated power P2 nj of the sendchannel j, and as a function of the N coupling variables COR(j,i), for ivarying from 1 to N, associated with the send channel j.
 27. A deviceaccording to claim 26, in which an adaptation step μ_(n)(i,j) for afilter Fij associated with a receive channel i and a send channel j isobtained from the following equation, in which b_(i) is a positiveconstant:${\mu_{n}( {i,j} )} = {\frac{P\; 1\; n_{i}}{{{b_{i} \cdot P}\; 1\; n_{i}} + {{{{COR}( {j,i} )} \cdot P}\; 2\; n_{j}} + {\sum\limits_{k \neq i}\;{{{{COR}( {j,k} )} \cdot P}\; 1\; n_{k}}}}.}$28. A device according to claim 26, in which the calculation of thecorrelation between the output signal Y1 n(j) and the input signal X2 n(i) is an envelope correlation calculation.
 29. A device according toclaim 28, in which the coupling variable COR(j,i) is a function of themaximum value Maxcor(j,i) of the correlation values corr_(ji)(d),calculated over a time window considered, each of the correlation valuescorr_(ji)(d) being calculated from the equation:${{coor}_{ji}(d)} = \frac{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{{n_{i}(c)} \cdot P}\; 2\;{n_{j}( {c + d} )}}}{\sum\limits_{c = 0}^{{LM} - 1}\;{P\; 1\;{n_{i}^{2}(c)}}}$in which c is a sampling time in the calculation time window of durationLM, d is a shift value between the input signal X2 n(i) and the outputsignal Y1 n(i), and P1 n _(j)(t) and P2 n _(j)(t) are respectively anestimate of the power of the input signal X2 n(i) and an estimate of thepower of the output signal Y1 n(j) at a time t.
 30. A device accordingto claim 29, in which the coupling variable COR(j,i) is linked to themaximum value Maxcor(j,i) of said correlation values corr_(ji)(d) by thefollowing equation, in which k is a positive constant:${{COR}( {j,i} )} = {\frac{k}{{Maxcor}( {j,i} )}.}$31. A device according to claim 26, in which each filter Fij associatedwith a receive channel i, and a send channel j generates a filteringsignal that is subtracted from the output signal Y1 n(j) to provide afiltered signal Y2 n(j), said device further comprising means forcalculating, for each send channel j, N second coupling variablesCOR2(j,i), for i varying from 1 to N, each of which being characteristicof the acoustic coupling between the filtered signal X2 n(j) from thesend channel and one of the N input signals X2 n(i), the adaptation stepμ_(n)(i,j) of an identification filter Fij associated with a receivechannel i and a send channel j being calculated also as a function ofsaid N second coupling variables COR2(j,i).
 32. A device according toclaim 31, in which an adaptation step μ_(n)(i,j) for a filter Fijassociated with a receive channel i and a send channel j is obtainedfrom the following equation, in which b_(i) is a positive constant:${\mu_{n}( {i,j} )} = {\frac{{COR}( {j,i} )}{{COR2}( {j,i} )} \cdot {\frac{{P1}\; n_{i}}{{b_{i} \cdot {P1n}_{i}} + {{{{COR}( {j,i} )} \cdot {P2}}\; n_{j}} + {\sum\limits_{k \neq i}\;{{{COR}( {j,k} )} \cdot {P1n}_{k}}}}.}}$33. A device according to claim 31, further comprising, for each paircomprising a receive channel i and a send channel j, gain applicationmeans for applying a receive gain Gr_(n)(i) to the input signal X2 n(i)and a send gain Ge_(n)(j) to the filtered signal Y2 n(j), said gainsGr_(n)(i), Ge_(n)(j) being calculated on the basis of the N secondcoupling variables COR2(j,i) determined for the send channel i.
 34. Anecho processing device for attenuating echo components of a directsignal X1 nin a return signal X2 n, said device comprising: means forcalculating a receive gain Gr_(n) and a send gain Ge_(n); first gainapplication means for applying the receive gain Gr_(n) to the directsignal and producing an input signal X2 n emitted into an echo generatorsystem; second gain application means for applying the send gain Ge_(n)to an output signal Y1 n from the echo generator system and producingthe return signal Y2 n; means for obtaining a first coupling variableCOR characteristic of the acoustic coupling between the direct signal X1n or the input signal X2 n and the output signal Y1 n, by calculatingthe correlation between the direct signal X1 n or the input signal X2 nand the output signal Y1 n, said gain calculation means being adapted tocalculate the receive gain Gr_(n) and the send gain Ge_(n) based on saidfirst coupling variable COR; and an echo canceller receiving at itsinput said input signal X2 n emitted into the echo generator system anda signal Y3 n from the echo generator system, the echo cancellercomprising: a finite impulse response identification filter whoseresponse is representative of the response of the echo generator system,the identification filter being adapted to generate a filtering signalSn and comprising means for subtracting the filtering signal Sn from thesignal Y3 n to produce said output signal Y1 n that is received at theinput of said second gain application means, means for adapting thecoefficients of the identification filter as a function of an adaptationstep μ_(n); and means for calculating the adaptation step μ_(n), saidadaptation step calculation means comprising means for estimating thepower P1 n of the input signal X2 n or the direct signal X1 n and thepower P3 n of the signal Y3 n, and means for obtaining a second couplingvariable COR2 characteristic of the acoustic coupling between the inputsignal X2 n or the direct signal X1 n, and the signal Y3 n from the echogenerator system, by calculating a correlation between the input signalX2 n or the direct signal X1 n, and the signal Y3 n; wherein theadaptation μ_(n) of the identification filter is calculated as afunction of the estimated powers P1 n, P3 n and as a function of saidsecond coupling variable COR2.
 35. An echo processing device accordingto claim 34, in which said adaptation step μ_(n) of the identificationfilter is calculated also as a function of the first coupling variableCOR.